The present invention relates to nuclear magnetic resonance spectroscopy and, more particularly, to a nuclear magnetic resonance spectrometer for detection of multiple quantum transitions.
In recent years, two-dimensional NMR has attracted interest as a new method of NMR. According to this new method, nuclear magnetic resonance signals are plotted as a two-dimensional spectrum, thus producing a higher resolution than conventional methods. That is, spectral lines are better split into multiplet patterns. This facilitates analyzing spectra. Hence, it is expected that the method will find much wider application.
In more recent years, a method of obtaining information about multiple quantum transitions, which was impossible for the prior art method to derive, by utilizing the above-mentioned two-dimensional NMR has been proposed as in the Journal of Magnetic Resonance, Vol. 48, pp. 158-163, 1982 and Chem. Phys. Lett. Vol. 52, No. 3, pp. 407-412, 1977. Referring to FIG. 1, this proposed method uses a pulse train consisting of three 90.degree. pulses: pulse P1 (non-selective pulse), pulse P2 (non-selective pulse), and pulse P3 (mixed pulse). The time period, .tau. is fixed. There is a phase difference .phi. between radio-frequency signals contained in the pulses P1 and P3. The resulting free induction decay signal FID is detected for a period of time t2 and stored in a memory. The application of the pulses P1 and P2 to a sample creates a disequilibrated statistical state of the rotating magnetic resonators contained in the sample. Thereafter, the mixed pulse P3 is applied which is shifted in phase by the phase angle .phi. with respect to the vibration characterizing the previous unequilibrium state of the resonators. The measurement is repeated while varying the period t1 in stepwise fashion. Further, each of these measurements is repeated with different values of the phase angle .phi.. The resulting free induction decay signals are stored in a memory, corresponding to the values of t1 and the values of .phi.. Then, linear combinations of the free induction decay signals are formed, and they are converted into the frequency domain by double Fourier transformation, thus producing a two-dimensional spectrum.
In the previously proposed multiple quantum NMR, free induction decay signals are detected with only one channel. In this case, only one frequency range that is higher or lower than the frequency of the irradiating RF pulses is measured, necessarily resulting in a reduction in the signal-to-noise ratio. Further, a memory having a large storage capacity is needed. Another problem arises from the fact that a peak of a spectrum which lies at a position corresponding to the irradiating frequency sees a 90.degree. pulse as it is, but peaks located away from the irradiating frequency do not see it as a 90.degree. pulse. The inevitable result is that the phase alters as the spectral position moves away from the irradiating frequency.
In an attempt to solve these problems, quadrature detection which has been adopted in the conventional nuclear magnetic resonance spectrometry differing from a two-dimensional nuclear magnetic resonance spectrometry may be contemplated. According to this, free induction decay signals are detected with two channels which are 90.degree. out of phase with each other, and the irradiating frequency is set to the center of a range to be measured. Accordingly, the two frequency ranges which are respectively higher and lower than the irradiating frequency can be separately measured. Therefore, if the storage capacity of the memory used is fixed, the resolution can be increased. If the resolution is retained constant, the storage capacity can be reduced. In addition, if the measured range is the same as in the case of using a single channel, the deviation from the irradiating frequency at each end of the measured range is half as compared with the single channel method. The phase shift accompanying the deviation from the irradiating frequency is decreased accordingly.
However, in the multiple NMR utilizing the aforementioned quadrature detection, the phase shift resulting from the deviation from the irradiating frequency decreases but does not become null. Further, if the phase difference between the two channels of detection system is not exactly 90.degree., if the gains of the two channels are not exactly identical, or if other imbalance between two devices exists, spectral peaks due to folding-back phenomenon or ghost signals may appear in positions of the obtained two-dimensional spectrum where no signals should exist, thus introducing obstacles to the analysis.